1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to an optical recording medium having wobbles formed on at least one lateral surface of grooves of a user data area and grooves of a lead-out area, and configured such that wobble characteristics are made different between the user data area and the lead-out area.
2. Description of the Related Art
In general, optical recording media are widely employed as information recording media for an optical pickup device for recording/reproducing information. The optical recording media are classified into read-only-memory (ROM) compact discs (CDs) and digital versatile discs (DVDs) according to information recording capacity. Further, a DVD disc capable of writing, erasing and reading information can be sub-divided into a digital versatile disc-random access memory (DVD-RAM) disc and a digital versatile disc-rewritable (DVD-RW) disc.
In such a DVD-RAM or DVD-RW disc, as shown in FIG. 1, there is a lead-in area 10 in which read only data, such as disc size, number of track layers on a readable plane or illegal copy preventing information, is recorded, a user data area 20 in which user data can be repeatedly read and/or written, and a lead-out area 30 in which other disc-related information is recorded.
As indicated by a portion “C” of FIG. 1, there are grooves 23 and lands 25 alternatively formed in the user data area 20, so as to perform recording and/or reproducing information marks 27 along a predetermined track. In FIG. 1, reference numeral 40 denotes a reproduction beam. From enlarged portions of the lead-in area 10 (“A”) and the lead-out area 30 (“B”), it is confirmed that physical pits 15, which is read only data, are formed thereon. Here, the lead-out area 30 performs various functions. For example, the lead-out area 30 guards an optical pickup so as not to deviate from a user data area while the optical pickup performs recording/reproduction.
In particular, as shown in FIG. 2, in a dual-layer optical recording medium having a first recording layer L0 and a second recording layer L1 of opposite track paths, the lead-out area 30 allows an optical pickup to keep performing tracking during interlayer jumping from the outermost circumference of the first layer L0 to the outermost circumference of the second layer L1 without deviating from the track paths. The opposite tracks are sequentially addressed from the inner circumference of the first recording layer L0 to the outer circumference thereof, and then from the outer circumference of the second recording layer L1 to the inner circumference thereof.
In a dual-layer ROM disc, an area serving as a lead-out area varies according to the reproduction method of a second layer. In case of a dual-layer ROM disc having opposite track paths, a middle area is separately provided at each of the outer circumferences of the first and second recording layers. However, in case of a rewritable optical recording medium, both pits and grooves can be used Therefore, in case of dual layer rewritable optical recording media, recording power is affected by the physical geometry of the first recording layer L0 during recording of data. In other words, when recording is performed on the second recording layer L1, a recording light beam passes through the first recording layer L0, resulting in a difference in the transmittance between pit portions and groove portions.
Light power was measured for an optical recording medium at a mirror area, a pit area, a groove area and a groove area with marks, for simulation of the light power depending on a difference in the transmittance according to various conditions of the first recording layer L0, as shown in FIGS. 3A through 3D. Here, the number of tracks trapped by laser beam transmitted through a lens was taken into consideration.
Tables 1 and 2 list input parameters and items for experimentation. In Table 1, Rc represents the reflectivity of a crystallized portion of a recording layer and Ra represents the reflectivity of an amorphous portion of a recording layer.
TABLE 1ParameterConditionWavelength (nm)400Numerical Aperture (NA)0.65/0.85Minimum mark length (μm)0.275/0.194ModulationEFM+ (Eight-to-Fourteen Modulation-plus)Track pitch (TP) (μm)0.30, 0.34, 0.38Reflectivity (%)Rc = 25, Ra = 5
TABLE 2ItemFactorExampleDual recording layerStructure of first recordingMirror, pits, grooves,layergrooves with marksHigh NANumber of tracks trapped85 for 0.65 of NAby laser beam160 for 0.85 of NAIncident angle of beam40.5° for 0.65 of NA58.2° for 0.85 of NA
FIG. 4 is a graph showing the measurement result of light power depending on the transmittance for the cases shown in FIGS. 3A, 3B, 3C and 3D. With reference to FIG. 4, according to the simulation result, a decrease in the light power is smallest in the mirror portion (graph line with solid squares), and the light power gradually decreases more in the order of a pit portion (graph line with solid diamonds), a groove portion (graph line with solid triangles block) and a groove mark portion (graph line with solid circles). Therefore, as shown in FIG. 4, in the case of a dual layer disc, the transmittance varies according to the physical geometry of the first recording layer L0, which in case of a rewritable optical recording medium can further affect the recording power during recording of data.